Summary1. There is a growing consensus that the distribution of species trait values in a community can greatly determine ecosystem processes and services delivery. Two distinct components of community trait composition are hypothesized to chiefly affect ecosystem processes: (i) the average trait value of the species, quantified by community-weighted mean trait values (CWM; related to the mass ratio hypothesis) and (ii) the degree to which trait values differ between species in a community, quantified by different indices of functional diversity (FD; related to non-additive community effects). The uncertainty on the relative effect of these two components is stimulating an increasing number of empirical studies testing their effects on ecosystem processes and services delivery. 2. We suggest, however, that the interdependence between CWM and FD poses a challenge on disentangling their relative importance. We present a framework that allows designing experiments to decouple and assess the effects of these two community functional components on ecosystem processes and services. To illustrate the framework, we focused on leaf litter decomposition, as this is an essential process related to important ecosystem services. Using simulations, we applied the framework for plant leaf litter traits (litter nitrogen and phenolic content) that are related to litter decomposition. 3. CWM and FD generally showed a hump-shaped relationship (i.e. at more extreme CWM values, communities can have only low FD values). Within this relationship, we showed that it is possible to select quasi-orthogonal combinations of CWM and FD that can be treated statistically. Within these orthogonal CWM and FD combinations, it is also possible to select species assemblages controlling for other community parameters, such as total biomass, total density and species richness. 4. Synthesis. The framework provides a novel approach for designing experiments to decouple the effects of CWM and FD of communities on ecosystem processes, which otherwise cannot be easily disentangled. To apply the framework and design proper experimental layouts, it is essential to have a priori knowledge of the key traits by which species affect ecosystem processes and service delivery.

1. It has long been recognized that leaf traits exert a crucial control on litter decomposition, a key process for nutrient cycling, and that invading species can greatly alter such soil processes via changes in mixed litter trait composition. Trait effects on ecosystem processes are hypothesized to operate via changes in either dominant trait values in the community (often calculated as community-weighted mean trait values; CWM) or trait functional diversity (dissimilarity between species trait values; FD). Few have studied the effects of these community trait components in tandem due to their interdependence. 2. We studied litter mixture decomposition using three exotic and six native European tree species with a range in litter decomposability, to disentangle the unique and combined roles of CWM and FD in explaining net litter mixture mass loss. 3. We showed that while CWM exerted the strongest effect on mass loss, FD modulated its effects, increasing mass loss in mixtures with low mean decomposability and decreasing mass loss in mixtures with high mean decomposability. Litter species identity and native/exotic status explained relatively little additional variation in mass loss after accounting for CWM and FD. We further showed that alterations to CWM and FD were more important than the replacement of a native species with an exotic counterpart in predicting mass loss. 4. Synthesis: Our results indicate that the effect of adding an exotic or losing a native species on litter decomposition rate can be predicted from how a species alters both CWM and FD trait values. This supports the idea that the repercussions of exotic species on ecosystem processes depends on the extent that introduced species bear novel traits or trait values and so on how functionally dissimilar a species is compared to the existing species in the community.

Recent investigations have shown that two components of community trait composition are important for key ecosystem processes: (i) the community-weighted mean trait value (CWM), related to the mass ratio hypothesis and dominant trait values in the community, and (ii) functional diversity (FD), related to the complementarity hypothesis and the divergence of trait values. However, no experiments controlling for the inherent dependence between CWM and FD have been conducted so far. We used a novel experimental framework to disentangle the unique and shared effects of CWM and FD in a leaf litter-macrodetritivore model system. We manipulated isopod assemblages varying in species number, CWM and FD of litter consumption rate to test the relative contribution of these community parameters in the decomposition process. We showed that CWM, but also the combination of CWM and FD, is a main factor controlling litter decomposition. When we tested individual biodiversity components separately, CWM of litter consumption rate showed a significant effect on decomposition, while FD and species richness alone did not. Our study demonstrated that (i) trait composition rather than species diversity drives litter decomposition, (ii) dominant trait values in the community (CWM) play a chief role in driving ecosystem processes, corroborating the mass ratio hypothesis, and (iii) trait dissimilarity can contribute in modulating the overall biodiversity effects. Future challenge is to assess whether the generality of our finding, that is, that dominant trait values (CWM) predominate over trait dissimilarity (FD), holds for other ecosystem processes, environmental conditions and different spatial and temporal scales.

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We attempt here to review recent studies focusing on droughts and hydrology in the Šumava Mts. The main question is can bark beetles affect water regimes in forest and what kind of measures might be taken -if any -to prevent bark beetle attacks. We compared results for natural forest, clear-cuts in managed forest and dead forest killed by a bark beetle attack. As expected, there was more water and a lower air temperature above the soil surface in natural forest. Dead trees shade undergrowth and so moderate temperature fluctuations. The conditions in clear-cuts are the worst for natural forest regeneration. There are no significant changes in the water cycle in catchment areas affected by bark beetle infections. However, it is predicted there is likely to be a slow decline in the amount of water due to a local change in climate, i.e. air temperature increase and precipitation decrease. It is concluded that droughts might occur more often and independently of bark beetle outbreaks in the future.

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The IPCC IS92a scenario predicts climate changes including within-year fluctuations in precipitation and a temperature increase of 1.7 °C by the year 2050 and a further 2.7 °C by the year 2100. We attempted to detect these changes in the Šumava Mts. and compare them with climate changes in the surrounding foothills. We used meteorological data records for the years 1961-2017, provided by the Czech Hydrometeorological Institute (CHMI). We recorded a decrease in precipitation, including snow cover, in the mountains and in the foothills during the last 15-20 years. Air temperature has also increased significantly in spring and summer over the last two decades. We assume that the increase in spring temperature negatively affects snow cover and causes it to melt earlier. We found that all these changes affect both the Šumava National Park and the surrounding foothills at the same rate; as a result, natural disturbances such as windstorm and bark beetle infestations occur more often and are more severe in both areas. Thus, changes in temperature and precipitation must be also considered in future management planning.

Proper management of woods infested by bark beetle-clearing infested trees to prevent spread of bark beetle, or leaving them to preserve biodiversity-is a hotly debated topic. Di erences in temperature regime between di erentially managed areas are often-used arguments in these discussions. Results from the eld measurements are confusing. Therefore, here we review previous studies and report our results of using thermal sensors in the eld to determine the factors that might a ect the di erences in temperature reported in previous papers. Our results indicate that the variability recorded in one particular habitat, dry forest, is associated with the speci c characteristics of the locality of each microsite/sensor. We conclude that it is important to consider not only the temperatures recorded but also describe microsites in detail in terms of vegetation structure, sunshine or numbers of trees per unit area.